The present invention relates in general to double containment storage tanks, and, more particularly, to a spill control system for double containment storage tanks.
In double wall low temperature liquid storage tanks, product can enter the annular space by any of the following ways: (1) overfilling the inner tank; (2) leakage through the wall of the inner tank; and (3) leakage through the bottom. In the event of leakage through the wall or through the bottom, the liquid will accumulate at the bottom of the annular space.
In known configurations of double wall low temperature liquid storage tanks, the annular space between the two walls is usually filled with granular insulation such as Perlite, or the like. As the liquid accumulates at the bottom of the annulus in a spill condition and progressively increases in height, that liquid will gain sensible heat, that is, stored heat within the body, from every component such liquid contacts, and this sensible heat gain is dependent on temperature difference between the liquid and the component in consideration, and the weight of the component. Due to this heat gain, the low temperature liquid will evaporate, thereby changing phase from liquid to gas. It is a known fact that gases need considerably more volume compared to the corresponding amount of liquid. Depending on the heat gain and the amount of liquid over-fill or leak, the vapor generation can be quite high. In the initial stage, all the components coming in contact with the cold liquid will go through a large temperature drop and therefore, initially, the rate of vapor generation will be very high. Thus, there is need for a system which is capable of handling large volumes of gas without damaging any parts of the tank.
It can be observed from past theory and experience in fluidizing granular material by gases that granular material offers very high resistance to the gas flow. This can be very dangerous to the inner tank, particularly if the cause of the product coming into the annulus is due to a leak from the bottom or wall of the inner tank at the same time that there is very little product in the tank. Resistance to gas flow can result in a pressure buildup under the inner tank bottom which can lift the bottom off its base and endanger the structural integrity of the inner tank.
As mentioned above, the pressure buildup can damage the bottom of the inner tank, but it can also damage the wall of the inner tank. The gas in the granular material can be in the form of small bubbles. In the case of small bubbles, the bubbles might rise through the granular material disturbing the state of that granular material. Small bubbles are almost unavoidable, but a point is reached where bubbles are too large, and, in the limit, the bubbles stretch from wall to wall of the container, that is, completely across the annulus, and act like pistons driving granular material ahead of them until they burst violently at the surface. This phenomenon is known as "slugging".
If slugging is approached when the inner tank is not full of product, the pressure buildup can exceed the external design pressure of the inner tank. Such pressure buildup may result in the buckling of the inner tank wall causing enormous damage.
Therefore, there is need for a system which prevents gas buildup in the annular space of double wall tanks in the event of a spill of liquid, especially low temperature liquid such as a cryogenic liquid, or the like, into the annulus of such double wall tank.